In recent years, along with a rising level of integration in semiconductor devices, circuit patterns of transistors and the like which configure the semiconductor devices are being increasingly miniaturized. Required in this miniaturization of the patterns is not simply a thinning of line width but also an improvement in dimensional accuracy and positional accuracy of the patterns. This trend applies also to semiconductor memory devices.
Conventionally known and marketed semiconductor memory devices such as DRAM, SRAM, and flash memory each use a MOSFET as a memory cell. Consequently, there is required, accompanying the miniaturization of patterns, an improvement in dimensional accuracy at a rate exceeding a rate of the miniaturization. As a result, a large burden is placed also on the lithography technology for forming these patterns which is a factor contributing to a rise in product cost.
In recent years, resistance varying memory is attracting attention as a candidate to succeed these kinds of semiconductor memory devices utilizing a MOSFET as a memory cell (refer, for example, to Patent Document 1). For example, there is known a resistance change memory (ReRAM: Resistive RAM) that has a transition metal oxide as a recording layer and is configured to store a resistance state in a nonvolatile manner.
Write of data to a memory cell is implemented by applying for a short time to a variable resistor a certain setting voltage Vset. As a result, the variable resistor changes from a high-resistance state to a low-resistance state. Hereinafter, this operation to change the variable resistor from a high-resistance state to a low-resistance state is called a setting operation.
In contrast, erase of data in the memory cell MC is implemented by applying for a long time to the variable resistor in the low-resistance state subsequent to the setting operation a resetting voltage Vreset which is lower than the setting voltage Vset of a time of the setting operation. As a result, the variable resistor changes from the low-resistance state to the high-resistance state. Hereinafter, this operation to change the variable resistor from a low-resistance state to a high-resistance state is called a resetting operation. The memory cell, for example, has the high-resistance state as a stable state (a reset state), and, in the case of binary data storage, data write is implemented by the setting operation which changes the reset state to the low-resistance state.
In order to determine whether a memory cell MC is in a high-resistance state or in a low-resistance state, this kind of conventional resistance change memory generates a reference current by a certain means and senses by a sense amplifier circuit a difference between the reference current and a cell current flowing through the memory cell. However, in the conventional resistance change memory, it is difficult to increase a margin between the distribution of resistance values of memory cells MC in a high-resistance state and the distribution of resistance values of memory cells MC in a low-resistance state for various reasons, which makes it difficult to set the reference current to an appropriate value. That is, the conventional scheme of determining data retained in a memory cell based on comparison between the reference current and a cell current cannot sufficiently reduce the possibility of erroneous data reading.